The overall purposes of the program project are (1) to characterize clinical features of people with Prader-Willi syndrome (PWS), as they relate to chromosomal and DNA-identified typologies; (2) to investigate specific behavioral perceptual, neuroanatomical, neuroendocrine, nutritional and electrophysiological features which may distinguish genetic/chromosomal subtypes of PWS; and (3) to test several neurochemical hypotheses concerning mechanisms underlying food motivation in PWS, while treating people with PWS neuropharmacologically. Fifty subjects with PWS will be recruited over a 5-year period, 40% of whom will be 12-17 years of age and 60% of whom are 18 years and older. The program project will compare features distinguishing the two major PWS typologies. A battery of psychobehavioral and clinical/laboratory tests will be conducted, including: constructing a genetic map of 15q; cognitive and affective (e.g., personality) assessments; ecobehavioral observations in the natural environment; radiological examination of bone (e.g., for osteoporosis); nutritional, metabolism and sleep assessments; and ophthalmological evaluation. The data from these analyses will be entered into a common data base making it possible to explore patterns of differences and similarities of the foregoing laboratory and clinical tests of persons falling into the two major genetic subcategories of PWS. Four projects will be conducted by investigators focusing on specific sub-issues relevant to the unique features of PWS. These investigators are as follows: Neurobehavioral pharmacology of food motivation in PWS Nutritional and metabolic characterization of PWS Visual perception in PWS Relational learning and food motivation in PWS In addition to the 50 subjects in the four projects, the Psychobehavioral Core will study a group of 20 children less than 12 years of age who will not be part of the rest of the protocol.